This invention relates to a mold for molding a disk-like recording medium such as an optical disk base (hereinafter referred to simply as "optical disk base").
In a conventional mold for molding an optical disk base such as that shown in FIGS. 6 and 7, a duplication stamper 12 having pit grooves in accordance with recorded information is supported on one of a pair of specular plates (on a movable specular plate 1 in the illustrated example), an inner peripheral portion of the stamper being fixed by an inner damper retainer 6 and an outer peripheral portion of the stamper being fixed by an outer stamper retainer 4. The pair of specular plates (the other of which corresponds to a stationary specular plate 2 in the illustrated example) facing each other are disposed to enable mold closing whereby a cavity 3 is formed. A molten resin is charged in the cavity 3 under pressure and is cooled and solidified to form a molded piece which is taken out of the mold by the mold opening operation of the specular plates 1 and 2. To enable the molded piece to be taken out, an annular air slit 24 is formed between the specular plate 2 and a bushing 23 disposed at the center of the specular plate 2, and another annular slit 9 is formed between the inner stamper retainer 6 attached to the specular plate 1 and a floating punch 8 disposed at the center of the retainer 6. When the molded piece is taken out, it is separated from a specular surface 2a of the specular plate 2 by compressed air supplied through the air slit 24 immediate before or at the time of mold opening. The molded piece is then separated from the stamper 12 by compressed air supplied through the air slit 9 and is thereafter pushed out by the floating punch 8, thus effecting mold release. The outer diametral end surface of the molded piece is defined by an inner peripheral surface of the outer stamper retainer 4 if no specific member for defining this surface is provided. In a case where a cavity ring is provided so that its portion is located inside the outer stamper retainer in the radial direction, the outer diametral end surface of the molded piece is defined by an inner peripheral surface of this ring. In a case where an annular slide member is provided on the side of one of the pair of specular plates to the other of which the stamper is attached and where the annular slide member is maintained in abutment with the stamper by the force of a spring or the like at the time of mold closing (this member having a function of limiting occurrence of burr and incapable of receding from the specular surface), the outer diametral end surface of the molded piece is defined by an inner peripheral surface of this member. (Examples of this arrangement are disclosed in Japanese Patent Unexamined Publication Nos. 62-105616 and 62-275722).
However, if a pressure remained immediately before mold opening is not uniformly applied to the outer diametral end surface of the molded piece, or if portions of the outer diametral end surface of the molded piece have different resistances to the mold release force, the compressed air supplied through the air slits 9 and 24 does not spread out smoothly and uniformly. In particular, separation of part of the molded piece at a position where the remaining pressure or the mold release resistance is larger is retarded, resulting in a deformation of the molded piece and, hence, a deterioration in the mechanical characteristics requisite for the optical disk base, e.g., warp or vibration of the recording surface.